1. Field of the Invention
The present invention relates to a thread cutting device of a sewing machine which serves to guide a thread caught by a thread catching member to a fixed blade and to cut the thread by the fixed blade.
2. Description of the Related Art
Conventionally, a thread cutting device of a sewing machine which serves to cut a thread such as a needle thread or a bobbin thread in the end of a sewing work over a horizontal shuttle (for example, see JP-B-3106472) is provided with a thread catching body 610 provided in a sewing machine bed and serving to receive a power from a lower shaft to be driven by a sewing machine motor, thereby carrying out forward and backward movements (see FIG. 20A). In the forward and backward movements of the thread catching body 610, a bobbin thread T is caught by a forked thread catching portion provided on a tip of the thread catching body 610 and is then guided to a fixed blade so that the bobbin thread T is cut.
In some cases, a servomotor is utilized as a motor to be employed as the sewing machine motor. In general, it is hard to control the servomotor in a low speed region. For this reason, it is difficult to set an operating speed of the thread catching body 610 interlocked with the lower shaft to be sufficiently low. In the case in which the bobbin thread T passed from a bobbin 240 is caught by the thread catching body 610, therefore, the bobbin thread T is pulled vigorously in the catching operation so that the bobbin 240 is excessively rotated and the bobbin thread T is excessively passed (see FIG. 20B). There is a problem in that sewing quality is deteriorated in a next sewing work due to the excessive pull-out.
In consideration of the problems, in recent years, there has also been developed a thread cutting device of a sewing machine having a structure in which a stepping motor is used as a driving source of the thread catching body 610 in a thread catching work to catch the bobbin thread T at a low speed and the bobbin thread T and a needle thread are cut by a great force through a driving force of the sewing machine motor in a cutting work.
It is preferable that the needle thread and the bobbin thread T which are connected to a cloth side after a cutting work carried out through the thread cutting device should have smaller lengths from the cloth to a thread end in order to have excellent finishing for a workpiece. When the length to the thread end connected to the cloth is long, an operator cuts the thread to be short by a hand. For this reason, a working efficiency is remarkably reduced. On the other hand, in the case in which the needle thread and the bobbin thread T which remain on a needle side of the sewing machine after the cutting work do not have proper lengths, the thread end slips from the needle at a next sewing start so that a seam may not be reliably formed. Therefore, a work for inserting the thread into the needle is required again so that the working efficiency is reduced. Furthermore, a hole is formed on the cloth through the needle. Consequently, quality of a workpiece is reduced.
A thread cutting device of a conventional sewing machine which serves to cut a thread by a thread catching body and a fixed blade has such a structure that the needle thread and the bobbin thread T are caught and cut in a close position to a needle hole of a throat plate. A length of a residual end of the thread remaining on the needle side after the cutting work is to be maintained. There is a problem in that a length of the residual end of the thread remaining on the cloth side after the cutting work is also increased correspondingly (see FIG. 21).